1. Field of the Invention
The present invention relates to a clamp circuit in an image signal transferring system.
2. Description of the Related Art
Conventionally, an image photographed by a camera, such as a video camera, is photoelectrically converted by an image sensor, before being outputted as an analog image signal. The analog image signal (i.e. video signal) is subjected to an A/D conversion to enable handling by a micro computer and related circuitry provided in the camera. The analog image signal includes a direct-current component and an alternating-current component. The direct-current component indicates an average brightness of a picture. The alternating-current component indicates detail information of the picture, and fluctuates about a base line, which corresponds to the direct-current component. Since the direct-current component is lost when the analog image signal passes through an alternating-current circuit, the direct-current component should be reproduced before the analog image signal is inputted to the A/D convertor. Therefore, it is necessary that a black level is fixed, namely, the black level is clamped to a predetermined level of the A/D convertor.
The clamp level of the black level fluctuates depending on an environment in which the camera is used, for example, the clamp level is sensitive to temperature. Unless the clamp level is adjusted every time, appropriate A/D conversion is not performed. Accordingly, whether the black level is appropriately clamped is checked by detecting an output of the A/D convertor with an extra D/A convertor, and thus the clamp level can be suitably regulated at all times.
However, the extra D/A convertor, for regulating the clamp level, causes the cost of the camera to be high.
Therefore, an object of the present invention is to provide a cost-effective, simply constructed clamp voltage generating circuit.
In accordance with an aspect of the present invention, there is provided a clamp voltage generating circuit comprising: a clamp circuit to which an analog image signal is inputted, the clamp circuit clamping a signal level of the analog image signal to a predetermined level during an optical black period of the analog image signal; an A/D convertor that converts the analog image signal outputted from the clamp circuit to a digital image signal; a low pass filter that smooths a voltage corresponding to a least significant bit of the digital image signal outputted from the A/D convertor, the low pass filter outputting the smoothed voltage; a sample-and-hold circuit that holds a portion of the smoothed voltage corresponding to a black level during the optical black period, the sample-and-hold circuit outputting the black level smoothed voltage; and a clamp level adjusting circuit that outputs a clamp voltage to the clamp circuit, based on a result of comparing the black level smoothed voltage and a standard voltage corresponding to a zero level of the digital signal of the black level outputted from the A/D convertor. A signal level at which the clamp circuit clamps is determined based on the clamp voltage.
While the black level smoothed voltage of the sample-and-hold circuit is higher than 0 volts, the clamp level adjusting circuit outputs a voltage generated by subtracting the black level smoothed voltage of the sample-and-hold circuit from the standard voltage as the clamp voltage; and when the black level smoothed voltage of the sample-and-hold circuit reaches 0 volts, the clamp level adjusting circuit pulls up the clamp voltage.
The clamp level adjusting circuit has a differential amplifier, a capacitor, a first resistance and a second resistance. A negative terminal of the differential amplifier is connected to the sample-and-hold circuit. An output terminal of the differential amplifier is connected to the capacitor and one terminal of the first resistance. Another terminal of the first resistance is applied with a predetermined voltage. A positive terminal of the differential amplifier is applied with the standard voltage through the second resistance.
When the black level smoothed voltage of the sample-and-hold circuit is higher than 0 volts, the differential amplifier subtracts the black level smoothed voltage of the sample-and-hold circuit from the standard voltage. When the black level smoothed voltage of the sample-and-hold circuit is 0 volts, the first resistance pulls up the clamp voltage.
The predetermined voltage, a resistance value of the first resistance and a resistance value of the second resistance are determined such that a voltage value of the output terminal of the differential amplifier, split by the first resistance and the second resistance, is higher than the standard voltage.
When the black level smoothed voltage of the sample-and-hold circuit is higher than 0 volts, the clamp voltage continues to be pulled-up below the standard voltage until the black level smoothed voltage of the sample-and-hold circuit reaches 0 volts; and when the black level smoothed voltage of the sample-and-hold circuit is 0 volts, the clamp voltage continues to be pulled-up until the black level smoothed voltage of the sample-and-hold circuit exceeds 0 volts.
In accordance with another aspect of the present invention, there is provided a method for adjusting a clamp level, when clamping an image signal, comprising: a first step in which a voltage corresponding to a least significant bit of a black level of an A/D converted signal is smoothed; and a second step in which the clamp level of the image signal is adjusted in accordance with the smoothed voltage so that the clamp level of the black level reaches 0 level of A/D conversion.
The second step includes: determining whether the smoothed voltage corresponding to the least significant bit of the black level exceeds 0 volts; and an adjustment so that the clamp level is lowered when the smoothed voltage of the black level exceeds 0 volts, and the clamp level is pulled up when the smoothed voltage is 0 volts.